1. Field of the Invention
The invention is directed to a roll tool for cold pilger rolling of pipes. The tool having a pair of rolls which can be moved in a reciprocating manner and roll along the intermittently stationary pipe and a roll mandrel which is held in the interior of the pipe. The rolls and roll mandrel are grooved so that the gap between the roll and the mandrel decreases continuously in a first reducing groove zone from the rough-pierced wall thickness to the finished wall thickness of the rolled pipe until reaching a minimum conicity of the two tools. The dimensional accuracy of the finished pipe is brought about in an adjoining smoothing groove zone with a slightly conical mandrel geometry.
2. Description of the Prior Art
Such roll tools are used to produce pipes from rough-pierced tube blanks with dimensional accuracy. In the smoothing groove, the dimensional accuracy of the finished pipe is brought about in that--ideally--the mandrel and roll groove are cylinders with a constant diameter in this region. Unfortunately, this ideal geometry can only be realized in the mandrel when copper is used as a working material for the pipe. Other materials require a slightly conical mandrel geometry in the smoothing groove.
In formerly conventional grooves with constant wall reduction in the entire reducing groove region, the different requirements for the amount of taper in the reducing groove and the smoothing groove result in kinks in both tools. As can readily be understood, such kinks are undesirable and should be avoided as far as possible. When smoothing out these kinks, i.e. grinding or polishing a transition, there is a risk that a necked-down portion will be produced in the mandrel by grinding and that the pipe will be rolled into this necked-down portion during the pilger rolling process. As a result, high longitudinal forces occur on the mandrel as the rolling process progresses, which can lead to destruction of the mandrel.